There has been a technology of forming a thin film transistor on a large area of an insulating substrate by using an amorphous silicon thin film or a polycrystalline silicon thin film. The thin film transistor is used in an active matrix type liquid crystal display device as the switching element for selecting a pixel electrode. Furthermore, a liquid crystal display device comprising a transistor array is also being introduced to the market; in which display device a driving circuit, among the peripheral circuits, consisted of polycrystalline silicon thin film transistors is incorporated besides the switching element made of thin film transistor.
A conventional method of manufacturing the thin film transistor is described below with reference to FIG. 2(a) through FIG. 2(f), which show the manufacturing process flow. As shown in FIG. 2(a), an amorphous silicon thin film, a polycrystalline silicon thin film or a single crystal silicon thin film (hereinafter these silicon thin films are referred to simply as silicon thin film) is formed on an insulating substrate 1, and an island region 2 for transistor is shaped using a resist pattern. On the surface of the island region 2, a gate insulation film 3 is formed, as shown in FIG. 2(b), and a gate electrode 4 is formed selectively thereon. As shown in FIG. 2(c), impurities ion 5 is implanted using the gate electrode 4 as mask to form a source region 6 and a drain region 7 in the island region 2 in a self-aligning manner.
As shown in FIG. 2(d), a first interlayer insulation film 8 is formed on the whole surface, and then first contact holes 9 are perforated through the first interlayer insulation film 8 and the gate insulation film 3 reaching, respectively, the source region 6 and the drain region 7. A metal film of silicon-containing aluminum or other material is deposited covering the whole surface of the first interlayer insulation film 8, which metal film is then patterned to form a metal interconnection layer 10 using a photo etching process.
As shown in FIG. 2(e), a second interlayer insulation film 11 is formed on the whole surface, and then a second contact hole 12 is perforated through the second interlayer insulation film 11 reaching the metal interconnection layer 10. A barrier metal 13 of a tungsten suicide or other such material is formed selectively on the second interlayer insulation film 11 including the second contact hole 12. In order to form a bonding pad, a first hole 14 for bonding pad is perforated through the second interlayer insulation film 11 reaching the metal interconnection layer 10.
As shown in FIG. 2(f), a third interlayer insulation film 15 is formed on the whole surface, and then a third contact hole 16 is perforated through the third interlayer insulation film 15 reaching the barrier metal 13, and a second hole 17 for bonding pad is formed in connection with the first hole 14 for bonding pad. A transparent electro-conductive film of ITO (Indium Tin Oxide) or the like is provided by deposition covering the whole surface. A photo-resist film is applied thereon, and etching is performed using the photo-resist film patterned selectively on a region of the first and the second holes 14, 17 for bonding pad and a region on the third interlayer insulation film 15 including the third contact hole 16. Etchant for etching the transparent electro-conductive film is made from a strong acid halogenide of hydrochloric acid group or iodic acid group. A transparent pixel electrode 18 and a bonding pad 19 are formed through the above-described procedure.
In the above-described conventional manufacturing method, the bonding pad 19 has been formed with a transparent electro-conductive film. Bonding of thin gold wire or other wires to the bonding pad 19 is not easy, as compared with a case where the bonding pad 19 is formed with aluminum film or a film of other metal.
Furthermore, when measuring the characteristics of a transistor by contacting a probe on bonding pad, the conventional bonding pad formed of a transparent electro-conductive film results in the measurement results of inferior accuracy because the contact resistance between a probe and the transparent electro-conductive film is higher as compared with a case where the latter is made of aluminum.